Insulating glass units (IGU's) are used in windows to reduce heat loss from building interiors during cold weather or to reduce heat gain in building interiors during hot weather. IGU's are typically formed by a spacer assembly that is sandwiched between glass lites. The spacer assembly usually comprises a frame structure that extends peripherally around the unit, an adhesive material that adheres the glass lites to opposite sides of the frame structure, and desiccant in an interior region of the frame structure for absorbing atmospheric moisture within the IGU. The glass lites are flush with or extend slightly outwardly from the spacer assembly. The adhesive is disposed on opposite outer sides of the frame structure about the frame structure periphery, so that the spacer is hermetically sealed to the glass lites. An outer frame surface that defines the spacer periphery may also be coated with sealant, which increases the rigidity of the frame and acts as a moisture barrier.
One type of spacer construction employs a U-shaped, roll formed aluminum or steel elements connected at its end to form a square or rectangular spacer frame. Opposite sides of the frame are covered with an adhesive (e.g., a hot melt material) for securing the frame to the glass lites. The adhesive provides a barrier between atmospheric air and the IGU interior. Desiccant is deposited in an interior region of the U-shaped frame element. The desiccant is in communication with the air trapped in the IGU interior and removes any entrapped water vapor and thus impedes water vapor from condensing within the IGU. After the water vapor entrapped in the IGU is removed, internal condensation only occurs when the seal between the spacer assembly and the glass lites fails or the glass lites are cracked.